1. Field of the Invention
The present invention relates to an apparatus and method for generating a clock synchronized with a reference clock by a PLL (Phase Locked Loop) method.
2. Description of the Prior Art
According to a conventional frequency synchronous method, degradation in the stability of a reference clock leads to degradation in the stability of the output frequency of a voltage-controlled oscillator (VCXO) following the reference clock. To solve this problem, there is provided an apparatus shown in FIG. 1. In this apparatus, a synchronous clock 109 output from a voltage-controlled oscillator 5 is branched and divided by a frequency division circuit 7. A frequency division circuit output 105 is compared with a reference clock 103 by a phase comparison circuit 4. A frequency adjustment and calculation circuit 1 receives phase difference data 106 from the phase comparison circuit 4, calculates such a control value as to eliminate the phase difference, and sends a control value 107 to a conversion circuit 3. The conversion circuit 3 sends a control voltage 108 to the voltage-controlled oscillator 5. Then, the voltage-controlled oscillator 5 outputs a synchronous clock 109 corresponding to the voltage. In this apparatus, a highly stable oscillator 52 is connected to the frequency adjustment and calculation circuit 1. The highly stable oscillator 52 monitors the reference clock, and restricts the follow-up performance of the synchronous clock output from the frequency synchronous apparatus to the reference clock.
Japanese Unexamined Patent Publication No. 5-268078 discloses an arrangement which holds a voltage value obtained when the reference clock and VCO clock synchronize with each other, and if the frequency difference between the reference clock and VCO exceeds the pulling range, switches the current voltage to the held voltage value to achieve phase locking. This arrangement enables quick recovery of re-synchronization when the frequency steps out.
However, the former apparatus requires the highly stable oscillator 52, which increases the circuit scale and cost. The latter apparatus is a means for re-synchronizing the synchronous clock with the reference clock when the synchronous clock changes. If the reference clock degrades, the current voltage is set to the held voltage value, and the voltage value of VCO is reset to a value used in synchronization. This inhibits a holdover state, and the frequency of the synchronous clock changes to a held frequency used in synchronization. That is, the synchronous clock disadvantageously coincides with the changed reference clock.
The present invention has been made to overcome the conventional drawbacks, and has as its object to provide a frequency synchronous apparatus and method capable of outputting a stable synchronous clock even when the frequency of a reference clock varies and the characteristics of a voltage-controlled oscillator change over time.
To achieve the above object, according to the first aspect of the present invention, there is provided a frequency synchronous apparatus for outputting a synchronous clock synchronized with a reference clock, comprising a switch which selects either one of a highly stable clock output and a reference clock output in accordance with a mode switching signal, a frequency division circuit which divides a frequency of the synchronous clock, a phase comparison circuit which detects a phase difference between an output clock from the frequency division circuit and an output clock from the switch, and outputs a phase difference value, a frequency adjustment and calculation circuit which performs synchronous control so as to adjust the phase difference value output from the phase comparison circuit to 0, and outputs a synchronous control value at this time, a memory which holds the synchronous control value output from the frequency adjustment and calculation circuit, a conversion circuit which converts the synchronous control value output from the frequency adjustment and calculation circuit into a control voltage value, and a voltage-controlled oscillator which outputs a synchronous clock on the basis of the control voltage value.
The memory in the first aspect holds a synchronous control value output from the frequency adjustment and calculation circuit when the highly stable clock is output in accordance with the mode switching signal, and two synchronous control values output from the frequency adjustment and calculation circuit when a frequency of the highly stable clock is set to a frequency higher by a predetermined value and to a frequency lower by the predetermined value.
To achieve the above object, according to the second aspect of the present invention, there is provided a frequency synchronous apparatus for outputting a synchronous clock synchronized with a reference clock, comprising a frequency division circuit which divides a frequency of the synchronous clock, a phase comparison circuit which detects a phase difference between an output clock from the frequency division circuit and a reference clock, and outputs a phase difference value, a frequency adjustment and calculation circuit which performs synchronous control so as to adjust the phase difference value output from the phase comparison circuit to 0, and outputs a synchronous control value at this time, a memory which holds a predetermined synchronous control value output from the frequency adjustment and calculation circuit, a conversion circuit which converts the synchronous control value output from the frequency adjustment and calculation circuit into a control voltage value, and a voltage-controlled oscillator which outputs a synchronous clock on the basis of the control voltage value.
The memory in the second aspect holds a synchronous control value output from the frequency adjustment and calculation circuit when a highly stable clock is input to the phase comparison circuit instead of the reference clock, and two synchronous control values output from the frequency adjustment and calculation circuit when a frequency of the highly stable clock is set to a frequency higher by a predetermined value and to a frequency lower by the predetermined value.
To achieve the above object, according to the third aspect of the present invention, there is provided a frequency synchronous control method of controlling a voltage-controlled oscillator to output a synchronous clock synchronized with a reference clock, comprising the steps of detecting a phase difference between the synchronous clock and the reference clock, performing synchronous control so as to eliminate the phase difference, controlling the voltage-controlled oscillator on the basis of a synchronous control value at this time, and holding the synchronous control value in a memory.
In the frequency synchronous control method of the third aspect, a synchronous control value which adjusts a phase difference value between the synchronous clock and a highly stable clock instead of the reference clock to 0, and two synchronous control values output which adjust the phase difference value from the synchronous clock to 0 when a frequency of the highly stable clock is set to a frequency higher by a predetermined value and to a frequency lower by the predetermined value can be held in the memory in advance.
In the frequency synchronous control method of the third aspect, when synchronous control is so performed as to eliminate the phase difference between the synchronous clock and the reference clock, whether a synchronous control value which adjusts the phase difference value to 0 is a value falling within a control range whose upper and lower limit values are the two synchronous control values held in the memory in advance can be checked, if the synchronous control value is the value falling within the control range, synchronous control which adjusts the phase difference value to 0 can be performed, and if the synchronous control value is a value falling outside the control range, a previous control value can be directly used.
In the frequency synchronous control method of the third aspect, on the basis of the synchronous control value held in the memory as a result of synchronous control which eliminates the phase difference between the synchronous clock and the reference clock, a control range of the voltage-controlled oscillator using the synchronous control value as a reference can be so corrected as to coincide with a control range whose upper and lower limit values are the two control values held in the memory in advance, and synchronous control values corresponding to new upper and lower limit values can be held in the memory.
In the frequency synchronous control method of the third aspect, when synchronous control is so performed as to eliminate the phase difference between the synchronous clock and the reference clock, whether the synchronous control value which adjusts the phase difference value to 0 is a value falling within a control range based on the new upper and lower limit values can be checked, if the synchronous control value is the value falling within the control range, synchronous control which adjusts the phase difference value to 0 can be performed, and if the synchronous control value is a value falling outside the control range, a previous control value can be directly used.
As is apparent from the above aspects, the present invention can output a stable synchronous clock within a predetermined range even if the stability of the reference clock degrades and the characteristic of the voltage-controlled oscillator changes over time.
The present invention does not require any oscillator for detecting the stability of the reference clock, and can reduce the cost.
In the present invention, the control range based on the clock of the highly stable oscillator is held in the memory in advance. A control value (control voltage value) when the synchronous clock synchronizes with the reference clock is periodically monitored, and the control range is corrected on the basis of this control value. The present invention can always output a stable synchronous clock even if the stability of the reference clock degrades or the characteristic of the voltage-controlled oscillator changes over time.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.